1. Field of the Invention
The present invention relates to a calibration method for an optical disc, and more particularly, to an optimal power calibration method for an optical disc of an optical system.
2. Description of the Prior Art
Along with the growing use of computers comes an increasing amount of digital data. One convenient way of storing this data is through the use of optical recorders, which include such devices as CD-RW drives. Optical recorders make use of a laser to write data onto an optical disc such as a CD-R disc. Writing data onto an optical disc is a very delicate process, and great care must be taken to prevent the disc from being ruined during the writing process. To help ensure the quality of the data stored on the optical disc, power calibration methods are used to calibrate the power of the laser that writes information onto the optical discs.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a CD-R 10 that is used during a write process. Only a ninety-degree sector of the CD-R 10 is shown for clarity. The CD-R 10 contains a center hole 12, a Power Calibration Area (PCA) 14, a Program Memory Area (PMA) 16, a lead-in area 18, a program area 20, and a lead-out area 22. In addition, the PCA 14 includes a count area and a test area. The count area has a plurality of counting units, and the test area has a plurality of test blocks that correspond to each of the counting units. The optical recorder uses these test blocks for writing test data onto the test blocks during the calibration process.
Please refer to FIG. 2 and FIG. 3. FIG. 2 is a flowchart of a method of optical power calibration according to the prior art. FIG. 3 is a chart illustrating the prior art method shown in FIG. 2. In the prior art method, the optical recorder reads an indicated power 40 from either the lead-in area 18 of the CD-R 10 or from a firmware database of the optical recorder, and then uses the indicated power 40 to calculate an optimal power 46. The laser is then supplied with the optimal power 46 when writing to the CD-R 10. The steps in the prior art calibration method are shown below.
step 30: read the indicated power 40 from the lead-in area 18; step 32: calculate 15 power levels (index values 0-14) that evenly cover a range 0.7*(indicated power) 44 to 1.3*(indicated power) 42, and use the 15 power levels to perform a test write; and step 34: calculate write performance of each of the 15 power levels, and use the write performance results to calculate the optimal power 46 through calculations involving interpolation or extrapolation.
As shown in FIG. 3, the 15 power levels have index values 0 to 14. The 15 power levels are evenly distributed, with index 0 laser power equal to 0.7*(indicated power) 44, index 14 laser power equal to 1.3*(indicated power) 42, and index 7 laser power equal to the indicated power 40. The optical recorder will perform test writes using each of these 15 power levels to help approximate exactly where the optimal power 46 lies. When the optimal power 46 is used to write to the CD-R 10, High Frequency (HF) signals are produced that have perfectly symmetric amplitudes. Each HF signal has an upper amplitude A1 and a lower amplitude A2. An amplitude measurement β is used to compare the relative sizes of A1 and A2, and is defined by the relationship β=(A1 A2)/(A1+A2). The optimal power 46 produces HF signals where the upper amplitude A1 is exactly equal to the lower amplitude A2, or β=0. By comparing β values of HF signals produced by test writes using each of the 15 power levels, an estimate of the optimal power 46 can be calculated.
Using FIG. 3 as an example, suppose that the optimal power 46 lies between index 8 laser power and index 9 laser power. The optical recorder then writes test data using each of the 15 power levels. When writing test data with the index 8 laser power, the lower amplitude A2 of the HF signal is greater than the corresponding upper amplitude A1, and β<0. On the other hand, when test writing with the index 9 laser power, the upper amplitude A1 of the HF signal is greater than the corresponding lower amplitude A2, and β>0. Thus, by analyzing the results of these two test writes, the optical recorder can calculate that the optimal power 46 lies between index 8 and index 9 laser powers. Interpolation can be used to accurately calculate the optimal power 46 by finding the laser power in which β=0.
However, problems can arise in the prior art method of calibrating optimal laser power. Every time the prior art calibration method is executed, each of the 15 power levels is used to perform a test write. Unfortunately, the CD-R 10 can be damaged by high power lasers during the test write process. For example, if the optimal power 46 is a lot lower than the indicated power 40, writing with index 15 laser power could destroy a surface of the CD-R 10 because the index 15 laser power is higher than the specification of the CD-R 10 allows. In this case, the CD-R 10 is ruined in the power calibration process, which is before the optical recorder even has a chance to write data to the program area 20 of the CD-R 10.